1. Field of the Invention
This invention relates to new and improved high power InGaAsP/InP semiconductor lasers having good optical characteristics with a very low series resistance that are able to operate at significantly increased operating currents and are able to produce higher output power than corresponding devices of the prior art. Accordingly, it is the general object of this invention to provide new and improved semiconductor lasers of such character.
2. General Background
For various applications, it is desirable to produce semiconductor lasers having as low a series resistance as possible so as to minimize the amount of heating that tends to occur during continuous-wave operation and to reduce the IR voltage drop which occurs across the cladding layer. The satisfaction of such a desideratum is particularly important for high power devices that operate at high currents, for very high frequency devices which operate at high CW bias, and for short cavity length devices which have much higher than normal series resistance due to reduced cavity length.
Typically, a conventional InGaAsP/InP semiconductor laser has a single p-doped InP layer associated therewith. Series resistance can be normally minimized by making the p-doped InP layer as thin as possible (1 to 2 microns) and as heavily p-doped as possible. Disadvantageously, however, in conventional InGaAsP/InP laser manufacture, when the p-InP layer is doped above 1.times.10.sup.18 atoms/cm.sup.3, the p-dopant (such as zinc) diffuses into the active layer, and producing increased threshold currents and decreased differential quantum efficiencies. See, for example, the following publications:
1. "Low Threshold Current Density (100) GaInAsP/InP Double Heterostructure Lasers for Wavelength 1.3 .mu. m", Y. Itaya, Y. Suematsu, S. Katayama, K. Kishino and S. Arai, Jap. J. Appl. Phys., Vol. 18, No. 9, pp. 1795-1805 (1979).
2. "InGaAsP Double-Channel-Planar-Buried-Heterostructure Laser Diode (DC-PBH LD) with Effective Current Confinement", I. Mito, M. Kitamura, K. Kobayashi, S. Murata, M. Seki, Y. Odagiri, H. Nishimoto, M. Yamaguchi and K. Kobayashi, J. Lightwave Tech., Vol. LT-1, No. 1, pp. 195-202 (1983).
3. "Fabrication, Characterization, and Analysis of Mass-Transported GaInAsP/InP Buried-Heterostructure Lasers", Z.L. Liau, J.N. Walpole, D.Z. Tsang, IEEE J. Quantum Electronics, Vol. QE-20, No. 8, pp. 855-865 (1984).
The reduction of the series resistance in the p-InP layer has various advantages. For example, ohmic heating is reduced. Also, the IR voltage drop which occurs across the p-doped InP layer can serve to turn-on either the PN InP homojunction which blocks shunt currents (as discussed by Z. Liau et al., cited hereinabove) or the PNPN current blocking layer (as suggested by H. Namizaki, R. Hirano, H. Higuchi, E. Oomura, Y. Sakakibara, W. Susaki, Electron. Letts., Vol. 18, No. 16, pp. 703-705 (1982). Thus, it is desirable to dope the p-InP region as highly as possible.
A number of studies have indicated that when the zinc concentration in the p-InP layer is above 1.times.10.sup.18 atoms/cm.sup.3, too much zinc diffuses into the active layer and the device properties actually become worse, as set forth, for example, in articles by Itaya et al. and Z.L. Liau et al., both cited hereinabove. The instant invention set forth hereinafter provides for a convenient means of increasing the zinc concentration in the p-InP layer to reduce the series resistance while simultaneously maintaining low zinc concentration in the active region.